Comparative Neuromuscular Laboratory


Masticatory muscle myositis and Neospora infection in a 4 month old female Cavalier King Charles Spaniel

Contributed by Dr. Simon Tappin and Dr. Kate Murphy Division of Companion Animals, Langford House, Langford, Bristol, UK

Clinical History

A 4 month old female Cavalier King Charles Spaniel (Figure 1) presented with a 10 day history of episodic facial swelling, progressive inability to open the jaw, and exopthalmia. During these episodes, the masticatory muscles appeared to swell and became firm and painful. On each occasion the swelling quickly decreased in response to corticosteroids (Dexamethasone 0.25mg/kg). However, clinical signs persisted and dysphagia resulted in weight loss. The dog had no prior history of illness, was fully vaccinated, wormed and had no history of travel outside the UK.

Figure 1

Figure 2 The full extent of jaw opening under general anesthesia

Physical and Neurological Examination

At presentation the dog was bright, alert and in fair body condition (weight 5.25Kg, condition score 2/5). Clinical examination revealed bilateral swelling of the masseter muscles, slight exophthalmia and discomfort on palpation of the masticatory muscle group and on retropulsion of both eyes. Jaw opening was limited to 15mm (Figure 2). Neurological examination was unremarkable.

Diagnostic Tests

Haematology - No significant abnormality detected
Serum biochemistry - Moderate elevations in CK and AST
Urinalysis - Unremarkable
Toxoplasma serology - IgG <50 and IgM <20, suggesting no active infection
Neospora serology (IFAT) - Positive 1:1600 ( Normal <50, Exposure <300)
Serum 2M Antibody titer (ELISA) - Positive 1:500 (reference <1:100)
Radiographs - Skull radiographs were unremarkable, the temporomandibular joints appeared normal. Thoracic radiographs were also unremarkable.
Electromyography - Fibrillation potentials were found in the masticatory muscles. All other muscle groups were unremarkable.

Fig. 3. Biopsy from the masseter muscle showing inflammation, extensive myofiber loss, edema, and fibrosis

Muscle biopsies - Biopsies were taken from the masseter and temporalis muscles. These revealed multifocal areas of mononuclear cell infiltration composed of scattered macrophages and clusters of lymphocytes, generalized myofiber loss and atrophy, fibrosis and edema (Figure 3).

Diagnosis and Further Outcome

Supportive care with intravenous fluids and syringe feeding was initiated and potentiated sulphonamides were also added (15mg/kg/BID). Ocular lubricants were used to prevent exposure keratitis in the left eye (Lacrilube overnight and Viscotears q4 hours in the day). Over the next 4 days the dog's demeanor improved, the nystamus resolved and the head tilt reduced in severity. The dog was discharged to her owner at this point.
Although the positive 2M antibody titre confirms a diagnosis of masticatory muscle myositis (MMM), the positive Neospora IFAT also indicates an active Neospora infection. The development of acute onset of neurological signs, lead us to believe there was active Neospora infection within the central nervous system. It was felt that there may also be Neospora infection within the muscles, however review of the muscle biopsies in light of this did not reveal any Neospora cysts or tachyzoites; antibody staining for Neospora was also negative. As masticatory muscle myositis is an immune mediated condition it is possible that the Neospora infection acted as a trigger for the development of 2M auto-antibodies.
Masticatory muscle myositis can lead to the rapid destruction of myofibers and fibrosis, and immunosuppressive therapy with corticosteroids is indicated. In this case steroids were not added initially due to the concern of worsening Neospora infection. Six weeks of clindamycin and potentiated sulphonamides were prescribed, and after 3 weeks, anti-inflammatory doses of corticosteroids were added. The dosage was incrementally increased to an immunosuppressive dose (1mg/kg/BID) whilst watching for worsening of the neurological signs. The dog remained stable and the jaw movement increased rapidly over the next 2 weeks. The prednisolone dose was then gradually reduced over the next 6 months.


The dog has now completed the 6 month tapering prednisolone course, and although a slight head tilt and the absence menace response are still present, the dog has a very good quality of life. The dog's jaw opens close to a normal range with normal prehension and chewing. A severe and early onset form of masticatory myositis is seen in Cavalier King Charles Spaniels, although this is yet to be fully characterized (see our July 2005 case of the month). With respect to the Neospora infection, this dog had never been outside the owners' garden, thus vertical transmission was suspected. The dog had been bred by the owner and the bitch was frequently walked on dairy farm land, a known risk factor for Neospora infection (Kramer et al, 2004); Neospora titres in the bitch did not indicate exposure to Neospora (IFAT <1:100 ), and there were no clinical signs of infection.


Kramer L, De Riso L, Tranquillo V M, Magnino S & Genchi C. Analysis of risk factors associated with seropositivity to Neospora caninum in dogs. Veterinary Record (2004) 154,692,693
Ruehlmann D, Podell M, Oglesbee M, Dubey J P. Canine Neosporosis: A case report and literature review. Journal of the American Hospital Association (1995) 31,174-183
Melmed C, Barton CL, Bergman R, Shelton G D. Masticatory muscle myositis: Pathogenesis, Diagnosis, and Treatment. Comp Continuing Ed Small Animal. August 2004

Page 'Breadcrumb' Navigation: